From Food to You: Nutrients, Digestion and Metabolism

01 / WHAT'S IN FOOD?

First, meet
the ingredients.

A meal is a mixture of molecules. Some supply fuel. Others build structures, help reactions happen, or provide the water those reactions happen in.

Start with the families and their roles. The atlas follows immediately, before we trace what happens after a meal. “Macro” and “micro” describe amounts needed, not molecule size or importance. “Essential” means a dietary supply is needed because the body cannot make enough.

LARGER AMOUNTS

Macronutrients

Carbohydrate, fat and protein supply energy and material. Water is also needed in large amounts, but supplies no calories. A food can contain several of these at once.

SMALLER AMOUNTS

Micronutrients

Vitamins are organic compounds; minerals are elements used in structures, fluid balance and reactions. Neither supplies calories. Small requirements can still have large consequences.

The chart below shows the families; the adjoining atlas supplies the individual names and roles. Essentiality is a separate question about dietary supply. [3]

THE FOOD FAMILY TREEFamilies → subgroups → familiar examples

An illustrated mixed meal of rice, lentils and vegetables. — HOW TO READ THE TREE
Lines connect families, subgroups or components—not reactions or measured portions. Colours distinguish families. **Essential** means a dietary source is needed because the body cannot make enough; it is a separate property.

Inspired by Dr. Andy Galpin’s nutrient overview, with classifications refined from the references below. The plate is an original illustration.

Read the complete food-family map

These are nutrition families and components. Essentiality describes dietary need; it is a separate property.

MacronutrientsWhat food contains

Needed in relatively large amounts: carbohydrates, fats, proteins and water. Water supplies no calories.

USDA National Agricultural Library: Macronutrients · CDC: About Water and Healthier Drinks

CarbohydratesMacronutrients

Sugars, starches and dietary fiber. Structure and digestibility are related, but they are not the same classification.

Cooper, The Cell: The Molecular Composition of Cells, 2000 · FDA: Questions and Answers on Dietary Fiber

SugarsCarbohydrates

Single sugars include glucose, fructose and galactose. Double sugars join two units: sucrose, lactose and maltose.

Monosaccharides: one sugar unit · Disaccharides: two linked units

Cooper, The Cell: The Molecular Composition of Cells, 2000

StarchCarbohydrates

A plant storage carbohydrate built from glucose. Amylose is mostly unbranched; amylopectin is branched.

Amylose · Amylopectin

Cooper, The Cell: The Molecular Composition of Cells, 2000

Dietary fiberCarbohydrates

Mostly carbohydrates that escape digestion in the small intestine. Definitions also include lignin, which is not a carbohydrate.

Cellulose · Pectin · Beta-glucans · Resistant starch

Some fiber is fermented by microbes. Soluble, viscous and fermentable describe different properties.

FDA: Questions and Answers on Dietary Fiber · National Academies: Proposed Definition of Dietary Fiber

Fats & other lipidsMacronutrients

Most dietary fat is triglyceride. Lipids also include phospholipids and cholesterol.

OpenStax Organic Chemistry: Waxes, Fats, and Oils · Cooper, The Cell: The Molecular Composition of Cells, 2000

TriglyceridesFats & other lipids

Glycerol joined to three fatty-acid chains. The three chains do not have to be identical.

OpenStax Organic Chemistry: Waxes, Fats, and Oils

Fatty-acid structureTriglycerides

Count carbon–carbon double bonds: saturated has none, monounsaturated has one, polyunsaturated has more than one.

Saturated · Monounsaturated · Polyunsaturated

Cis and trans describe double-bond geometry. Omega numbering describes position; these are additional labels, not rival categories.

OpenStax Organic Chemistry: Waxes, Fats, and Oils · FDA: Trans Fatty Acids in Nutrition Labeling

Essential fatty acidsFats & other lipids

Linoleic acid (LA, omega-6) and alpha-linolenic acid (ALA, omega-3) require a dietary source.

LA — linoleic acid · ALA — alpha-linolenic acid

EPA and DHA are other omega-3s. Conversion from ALA is limited; 'omega-3' is a family name, not one molecule.

NIH ODS: Omega-3 Fatty Acids

Other lipid structuresFats & other lipids

Phospholipids help form membranes. Cholesterol is a sterol with a ring structure, not a triglyceride.

Cooper, The Cell: The Molecular Composition of Cells, 2000

ProteinsMacronutrients

Chains of amino acids that form working molecules throughout the body.

Alberts et al.: The Shape and Structure of Proteins, 2002 · MedlinePlus: Amino Acids

9 essential amino acidsProteins

These require a dietary supply because the body cannot make enough of them.

Histidine · Isoleucine · Leucine · Lysine · Methionine · Phenylalanine · Threonine · Tryptophan · Valine

MedlinePlus: Amino Acids

Amino acids we can makeProteins

Often called nonessential amino acids. 'Nonessential' describes synthesis, not whether the body uses them.

Alanine · Asparagine · Aspartate · Glutamate

MedlinePlus: Amino Acids

When synthesis is insufficientProteins

Some ordinarily synthesizable amino acids can become conditionally essential in particular physiological or illness states.

Arginine · Cysteine · Glutamine · Tyrosine

Context matters. This is not a fixed extra food group or a reason to prescribe a supplement.

MedlinePlus: Amino Acids

WaterMacronutrients

The fluid medium for chemistry and transport. Needed in large amounts, with zero calories.

CDC: About Water and Healthier Drinks · USDA National Agricultural Library: Macronutrients

Vitamins & mineralsWhat food contains

Required in smaller quantities. They support reactions, structures and regulation without supplying calories.

MedlinePlus: Vitamins · MedlinePlus: Minerals

VitaminsVitamins & minerals

Thirteen recognized vitamins, grouped here by solubility.

MedlinePlus: Vitamins

Fat-solubleVitamins

Vitamins A, D, E and K.

A · D · E · K

MedlinePlus Medical Encyclopedia: Vitamins

Water-solubleVitamins

Vitamin C and the eight B vitamins.

C · B1 · B2 · B3 · B5 · B6 · B7 · B9 · B12

Water-soluble does not mean unstored: B12 has substantial body stores.

MedlinePlus Medical Encyclopedia: Vitamins · NIH ODS: Vitamin B12

MineralsVitamins & minerals

Elements used in structures, ions and many biochemical systems. Major and trace refer to required quantities.

MedlinePlus: Minerals

Major mineralsMinerals

Calcium, phosphorus, magnesium, sodium, potassium, chloride and sulfur.

Calcium · Phosphorus · Magnesium · Sodium · Potassium · Chloride · Sulfur

Sulfur is supplied mainly through sulfur-containing amino acids; this is not a separate elemental sulfur target.

MedlinePlus: Minerals · National Academies: Sulfate

Trace mineralsMinerals

Established dietary roles include iron, zinc, copper, iodine, selenium, manganese and molybdenum.

Iron · Zinc · Copper · Iodine · Selenium · Manganese · Molybdenum

MedlinePlus: Minerals · NIH ODS: Molybdenum

A few classification detailsMinerals

Chromium's essentiality is disputed. Fluoride benefits teeth but is not essential for growth or survival. Cobalt is required within vitamin B12.

NIH ODS: Chromium · EFSA: Dietary Reference Values for Fluoride · NIH ODS: Vitamin B12

CholineWhat food contains

An essential nutrient used in membranes, acetylcholine and methyl-group metabolism. It sits outside the conventional 13-vitamin list.

NIH ODS: Choline

Other plant compoundsWhat food contains

Often called phytochemicals. Many are biologically active without being established essential nutrients.

Polyphenols · Lycopene · Lutein

This grouping can overlap nutrient chemistry: some carotenoids can be converted into vitamin A.

NCI Drug Dictionary: Phytochemical · NCI Dictionary of Cancer Terms: Polyphenol · NIH ODS: Vitamin A and Carotenoids

THE SHORT VERSIONA meal is a mixture.Watch the 24-second recap

Silent recap · A mixed meal supplies several nutrient families. Digestion opens different routes; cells can build structures, store material and transfer energy. Created with Shreyam’s code2video project.

Read the video transcript

A meal is a mixture. Foods contain several nutrients at once. Lentils, for example, contain starch, protein and fiber; the illustration does not show measured amounts.

Digestion opens different routes. Digestible carbohydrates yield simple sugars. Proteins yield amino acids. Fats yield fatty acids and monoacylglycerols. Some fiber reaches gut microbes.

Cells can build, store and transfer energy. They can make working structures, keep material for later and regenerate ATP to power cellular work. Follow each route in the full article.

02 / THE NUTRIENT ATLAS

The machinery needs
more than fuel.

Vitamins and minerals help reactions, structures and signals work. They supply no calories themselves.

Fat-soluble vitamins A, D, E and K help with roles such as vision, calcium regulation, antioxidant protection and clotting. The B vitamins support many enzyme reactions; vitamin C supports collagen synthesis. Minerals help build structures, carry oxygen and maintain electrical and fluid balance. Each tile below explains the individual nutrient.[3][31][36]

The family chart and this table are two scales of the same map. You do not need to memorize every tile to follow the story. Continue to the overview →

The atlas brings the pieces together: vitamins, minerals, water, choline, essential amino acids and essential fatty acids. One table places them alongside food families, molecular building blocks and special cases. Each tile explains its place in the picture and links to its sources.

THE BUILDING BLOCKS OF NUTRITION

The nutrient atlas.

CARBON ATLAS
PLATE 01 · 2026

Open a tile for its forms, food sources and journey through the body. Colors group related chemistry; labels distinguish dietary essentials, families, examples and context.

82 entries · one table

01–82 Atlas referencesESS Dietary essentialFAM Broader familyPART Molecular example / building blockNOTE Context matters

All entries are shown. Search highlights matches without moving the tiles.

01Water & choline

Two nutrients that deserve their own place.

Water & choline

Water + choline

02Carbohydrates

Single sugars → joined sugars → larger structures.

Carbohydrates

One sugar unit

Carbohydrates

Two sugar units

Carbohydrates

Broader families

Carbohydrates

Body store

03Fats & lipids

Structure, double-bond count and geometry are different ways to sort.

Fats & lipids

Molecular structure

Fats & lipids

Double-bond count

Fats & lipids

Bond geometry

Fats & lipids

Essential fatty acids

04Proteins & amino acids

Twenty standard building blocks. Many possible sequences.

Proteins & amino acids

The family

Proteins & amino acids

Nine indispensable amino acids

Proteins & amino acids

Eleven the body can usually make

Proteins & amino acids

The qualification

05Vitamins

Thirteen vitamin entries, grouped by solubility.

Vitamins

Fat-soluble · A, D, E, K

Vitamins

Water-soluble · eight B vitamins + C

06Minerals & electrolytes

“Major” and “trace” refer to quantities, not importance.

Minerals & electrolytes

Major dietary minerals

Minerals & electrolytes

Established trace minerals

Where do salts and electrolytes fit?

Salt is a compound; an electrolyte carries charge in solution. Sodium, potassium, chloride, calcium, magnesium and phosphate have electrolyte roles. Phosphorus is the nutrient label; phosphate is an ionic form. Bicarbonate is another body electrolyte, not an extra essential mineral. Reference

07Plant compounds

Selected families in food, with overlapping chemistry.

Plant compounds

Phytochemical families

08Special cases & body-made compounds

Useful connections, with their qualifications kept visible.

Special cases & body-made compounds

Elemental context

Special cases & body-made compounds

Made in the body

Read the family notes Why some labels overlap

Carbohydrates

Length is one question.

Digestion is another. Fiber is not simply the longest chain; starch can be very long too. Reference

Fats & lipids

Labels can overlap.

LA and ALA are both polyunsaturated. Omega-6 and omega-3 locate the first double bond from the methyl end: another way to classify fatty acids. Reference

Proteins & amino acids

9 + 11 = 20.

“Essential” describes dietary supply. “Nonessential” does not mean unimportant. Some needs depend on the physiological setting. Reference

Minerals & electrolytes

Where do salts and electrolytes fit?

Salt is a compound; an electrolyte carries charge in solution. Sodium, potassium, chloride, calcium, magnesium and phosphate have electrolyte roles. Phosphorus is the nutrient label; phosphate is an ionic form. Bicarbonate is another body electrolyte, not an extra essential mineral. Reference

Plant compounds

A family can cross the map.

Phytosterols are also lipids. Some carotenoids supply vitamin A. Polyphenols include flavonoids, stilbenes and lignans. These are selected families, not a list of universal dietary essentials.

Special cases & body-made compounds

The label matters.

Sulfur and cobalt occur within other required nutrients. Fluoride has a dental benefit; chromium’s essentiality is disputed. A body-made molecule can be important without being a separate dietary requirement.

A nutrition map inspired by the layout of a periodic table. Positions are editorial; they do not represent atomic numbers or chemical periodicity. Corner numbers locate entries in this atlas. Mineral tiles use element symbols; the others use nutrient abbreviations. These are 82 learning entries, not 82 separate dietary requirements. ESS marks a needed dietary contribution, sometimes alongside synthesis or precursors.

Keep the atlas data

Read every entry as text 82 entries

Coverage: adult nutrition, with selected food families and classification notes. No intake targets or supplement recommendations. “Major” and “trace” describe required quantities, not importance. More is not automatically better.

Water & choline

Two essential nutrients outside the conventional vitamin and mineral lists. Water is needed in large amounts and supplies no calories.

Water Essential nutrient

Forms body fluids, transports dissolved substances, lubricates tissues and supports temperature regulation. Water comes from drinks, food and metabolic reactions. Water is a macronutrient by quantity, but it does not supply calories.

What the name includes. Water is the same H2O molecule in a glass or within food; this macronutrient provides volume without supplying calories.

Where it appears. Plain water, milk, soup, tea and the water contained in foods all contribute to total water intake.

How the body handles it. Water forms body fluids, carries dissolved substances and lubricates tissues; evaporation of sweat helps regulate body temperature.

Reference for Water

Choline Essential nutrient

Used to make phosphatidylcholine, sphingomyelin and acetylcholine; also contributes methyl groups and supports lipid transport. The body makes some choline, but normally needs a dietary contribution. The body makes some choline, but dietary choline is still needed. It is not a fourteenth vitamin.

What the name includes. Food supplies free choline and several choline-containing compounds, including water-soluble forms and the phospholipids phosphatidylcholine and sphingomyelin.

Where it appears. Eggs, fish, dairy, soybeans, other beans and cruciferous vegetables are examples of foods that contribute choline.

How the body handles it. Water-soluble forms enter portal blood toward the liver, while some intact fat-soluble forms enter chylomicrons and travel through lymph.

Reference for Choline

Fat-soluble vitamins

A, D, E and K are the four fat-soluble vitamins. One vitamin entry may include several related chemical forms.

Vitamin A Essential vitamin

Retinoids support vision, immune function and the growth and differentiation of cells. Retinal participates in rhodopsin, the light-sensitive protein in the retina.

What the name includes. Vitamin A includes preformed retinoids and provitamin A carotenoids, such as beta-carotene, which the body can convert into vitamin A.

Where it appears. Eggs, dairy and fish supply preformed vitamin A; carrots, sweet potatoes and leafy greens supply provitamin A carotenoids.

How the body handles it. Much of the body's vitamin A reserve is stored in the liver as retinyl esters for later use.

Reference 1 for Vitamin A · Reference 2 for Vitamin A · Reference 3 for Vitamin A

Vitamin D Essential vitamin

After activation, supports calcium absorption and calcium–phosphate balance for bone mineralization. Calcitriol is the active form, produced mainly through liver and kidney processing. Vitamin D can also be synthesized in skin after ultraviolet exposure; it is still classified among the 13 vitamins. Its role is not evidence that extra intake always helps.

What the name includes. Vitamin D has two main dietary forms, D2 (ergocalciferol) and D3 (cholecalciferol), which follow the body's activation pathway.

Where it appears. Fatty fish provide vitamin D, with smaller amounts in egg yolks; some milks, plant drinks and cereals are fortified.

How the body handles it. The liver first converts vitamin D into calcidiol; a second conversion, mainly in the kidneys, produces the active hormone calcitriol.

Reference for Vitamin D

Vitamin E Essential vitamin

A fat-soluble antioxidant that helps protect cells from oxidative damage. Alpha-tocopherol is the form recognized to meet human vitamin E needs.

What the name includes. Natural vitamin E comprises eight tocopherol and tocotrienol forms; alpha-tocopherol is the form recognized as meeting human vitamin E requirements.

Where it appears. Nuts, seeds and vegetable oils supply vitamin E, with additional contributions from leafy greens and some fortified cereals.

How the body handles it. After intestinal absorption, the liver preferentially returns alpha-tocopherol to circulation while metabolizing and excreting other vitamin E forms.

Reference for Vitamin E

Vitamin K Essential vitamin

Required for modifying proteins involved in blood clotting and bone metabolism. Vitamin K supports gamma-carboxylation of selected glutamate residues.

What the name includes. Vitamin K includes K1 (phylloquinone) and a family of K2 forms called menaquinones, which differ in their side chains.

Where it appears. Leafy greens and some vegetable oils supply K1; certain fermented foods and animal foods contribute differing amounts of K2.

How the body handles it. Following intestinal absorption, vitamin K travels in lipoproteins; its tissue reserves are relatively small compared with other fat-soluble vitamins.

Reference for Vitamin K

Water-soluble vitamins

Eight B vitamins and vitamin C. Water-soluble does not mean unstored; vitamin B12 has substantial body stores.

Thiamin Essential vitamin

Its active form helps enzymes handle glucose, fats and amino acids. Thiamin diphosphate (TDP), also called thiamin pyrophosphate (TPP).

What the name includes. Thiamin occurs free or with phosphate groups attached; thiamin diphosphate, also called TPP, is its main active cofactor form.

Where it appears. Whole grains, pork, fish and other meats contain thiamin; enriched or fortified breads and cereals can also contribute.

How the body handles it. Digestion releases free thiamin before small-intestinal uptake; the body keeps only small reserves, principally in the liver.

Reference for Thiamin

Riboflavin Essential vitamin

Provides the vitamin component of FAD and FMN, cofactors used in energy metabolism. FAD and FMN participate in oxidation–reduction reactions.

What the name includes. Riboflavin occurs free or within FMN and FAD, the vitamin-containing cofactors that help enzymes transfer electrons during metabolism.

Where it appears. Milk, eggs and lean meats provide riboflavin, along with some vegetables and grain products enriched or fortified with it.

How the body handles it. Riboflavin is absorbed mainly in the first part of the small intestine, with small reserves in the liver, heart and kidneys.

Reference for Riboflavin

Niacin Essential vitamin

Precursor to NAD and NADP, used in fuel breakdown, biosynthesis and cellular maintenance. NAD⁺/NADH and NADP⁺/NADPH transfer reducing equivalents.

What the name includes. Niacin is a collective name for nicotinic acid, nicotinamide and related compounds that can supply the vitamin's activity.

Where it appears. Poultry, beef, fish, nuts, legumes and grains supply niacin; enriched and fortified grain products are additional sources.

How the body handles it. Absorbed niacin is rebuilt into NAD and NADP; the liver can also make NAD from the amino acid tryptophan.

Reference for Niacin

Pantothenic acid Essential vitamin

Needed to make coenzyme A and acyl carrier protein, which carry carbon groups during metabolism. The CoA in acetyl-CoA depends on vitamin B5.

What the name includes. Much food vitamin B5 is embedded in coenzyme A and related compounds; digestion releases pantothenic acid for reuse.

Where it appears. Mushrooms, poultry, eggs and whole grains are among the many plant and animal foods that supply pantothenic acid.

How the body handles it. After intestinal absorption, tissues use pantothenic acid to rebuild coenzyme A, supporting both fatty-acid breakdown and fatty-acid synthesis.

Reference for Pantothenic acid

Vitamin B6 Essential vitamin

Active B6 cofactors support amino-acid reactions, glycogen breakdown and neurotransmitter production. Pyridoxal phosphate (PLP) supports amino-group transfer.

What the name includes. Vitamin B6 is a family of six related forms, including pyridoxine; PLP and PMP are its active enzyme helpers.

Where it appears. Fish, poultry, chickpeas, potatoes and noncitrus fruits provide vitamin B6, and some breakfast cereals have it added.

How the body handles it. Phosphate groups are removed before uptake in the jejunum; cells use B6 cofactors for amino-acid reactions and glycogen breakdown.

Reference for Vitamin B6

Biotin Essential vitamin

Cofactor for carboxylases in glucose, fatty-acid and amino-acid metabolism. Pyruvate carboxylase and acetyl-CoA carboxylase use biotin.

What the name includes. Biotin occurs as free vitamin or attached to food proteins; digestion must release the bound vitamin before absorption.

Where it appears. Cooked eggs, fish, seeds, nuts and sweet potatoes are examples of foods contributing biotin to a mixed diet.

How the body handles it. Digestive enzymes release protein-bound biotin before small-intestinal absorption; the liver holds much of the body's stored biotin.

Reference for Biotin

Folate Essential vitamin

Folate cofactors carry single-carbon units needed for DNA production and amino-acid metabolism. Tetrahydrofolate derivatives support nucleotide synthesis.

What the name includes. Folate describes a family of related compounds; natural food folates differ chemically from folic acid commonly added during fortification.

Where it appears. Leafy greens, beans, peas and asparagus supply natural folates, while some breads and cereals contain added folic acid.

How the body handles it. Food folates are trimmed before intestinal absorption; circulating folate is mainly 5-MTHF, and the liver holds a substantial reserve.

Reference for Folate

Vitamin B12 Essential vitamin

Supports nervous-system function, red-blood-cell formation and DNA synthesis. Cofactor for methionine synthase and methylmalonyl-CoA mutase.

What the name includes. Vitamin B12 comprises cobalt-containing cobalamins; methylcobalamin and adenosylcobalamin are the two forms cells use as enzyme cofactors.

Where it appears. Fish, meat, eggs and dairy contain B12; some breakfast cereals and nutritional yeasts are fortified with it.

How the body handles it. Intrinsic factor normally enables uptake in the terminal ileum; despite being water-soluble, B12 can have body stores lasting years.

Reference for Vitamin B12

Vitamin C Essential vitamin

Required for collagen and carnitine synthesis; acts as an antioxidant and helps absorb nonheme iron. Ascorbate supports collagen-building enzyme reactions.

What the name includes. Vitamin C is ascorbic acid, also called ascorbate in its ionized form; it is a water-soluble organic nutrient.

Where it appears. Citrus fruits, bell peppers, kiwifruit, strawberries and broccoli are familiar examples of foods that contribute vitamin C.

How the body handles it. The body controls vitamin C through intestinal absorption and kidney excretion; vitamin C also improves absorption of nonheme iron.

Reference for Vitamin C

Major dietary minerals

Six established dietary mineral entries, needed in comparatively larger amounts. Sulfur has a separate special-case tile because it arrives mainly within other nutrients.

Calcium Essential mineral

A major part of bones and teeth; calcium ions also help muscles contract and cells communicate. Intracellular calcium is a signal, while most body calcium is stored in the skeleton.

What the name includes. Calcium occurs in mineral compounds, including bone's calcium phosphate, and as dissolved ions involved in signaling and muscle contraction.

Where it appears. Milk, yogurt, calcium-set tofu, fish with edible bones and some fortified plant drinks are examples of calcium sources.

How the body handles it. Vitamin D supports active calcium absorption in the intestine; bones and teeth hold almost all of the body's calcium.

Reference 1 for Calcium · Reference 2 for Calcium

Phosphorus Essential mineral

Present in bones, DNA, RNA, membranes and ATP. Phosphate transfer helps regulate enzymes. ATP is adenosine triphosphate: its name includes its three phosphate groups.

What the name includes. Food phosphorus occurs in phosphates and organic compounds; plant seeds also hold some in phytate, which people digest less effectively.

Where it appears. Dairy, eggs, meat, fish, beans, nuts and whole grains are among the many foods that provide phosphorus.

How the body handles it. Phosphorus is absorbed in the small intestine; the kidneys, intestines and bones coordinate phosphate balance across the body.

Reference 1 for Phosphorus · Reference 2 for Phosphorus

Magnesium Essential mineral

Supports enzyme systems in glycolysis, oxidative phosphorylation and protein synthesis. Many reactions using ATP require magnesium.

What the name includes. Magnesium, symbol Mg, is a mineral used as charged ions in enzyme chemistry, including reactions involving ATP.

Where it appears. Leafy greens, beans, lentils, nuts, seeds and whole grains supply magnesium, with variable contributions from drinking water.

How the body handles it. Bone holds much of the body's magnesium, with most of the remainder in soft tissues; kidneys regulate urinary losses.

Reference 1 for Magnesium · Reference 2 for Magnesium

Sodium Essential mineral

Helps maintain body-fluid balance and normal nerve and muscle function. A major positively charged ion outside cells.

What the name includes. Sodium occurs in several food compounds; familiar table salt is sodium chloride, while baking soda supplies sodium as bicarbonate.

Where it appears. Table salt, soy sauce, breads, cheeses and prepared soups can supply sodium, with smaller natural contributions from foods such as milk.

How the body handles it. The kidneys adjust how much sodium leaves in urine, helping control fluid balance alongside sodium's roles in nerve and muscle function.

Reference 1 for Sodium · Reference 2 for Sodium · Reference 3 for Sodium · Reference 4 for Sodium

Potassium Essential mineral

Helps maintain fluid inside cells and the gradients used by nerves and muscles. The sodium–potassium pump uses ATP to maintain ion gradients.

What the name includes. Potassium, symbol K, is a mineral electrolyte supplied by different food compounds; it is distinct from vitamin K.

Where it appears. Beans, lentils, potatoes, fruits, vegetables, milk and yogurt are examples of foods contributing potassium to a varied diet.

How the body handles it. Most body potassium is inside cells; intestinal absorption and kidney-controlled urinary losses help maintain the balance needed for electrical signaling.

Reference 1 for Potassium · Reference 2 for Potassium

Chloride Essential mineral

Helps maintain body-fluid balance and is a component of stomach digestive juices. Table salt provides sodium and chloride.

What the name includes. Chloride is an electrolyte supplied by salts such as sodium chloride and potassium chloride, with the salts separating into dissolved ions.

Where it appears. Table salt and foods made with it contribute chloride, which also occurs in foods such as tomatoes, celery and olives.

How the body handles it. Chloride helps balance body fluids and forms part of stomach acid; the body can remove excess chloride in urine.

Reference 1 for Chloride · Reference 2 for Chloride · Reference 3 for Chloride

Established trace minerals

Seven established trace-mineral requirements. Trace describes the small quantity needed, not a smaller biological importance.

Iron Essential mineral

Iron in hemoglobin carries oxygen from lungs to tissues; myoglobin supports oxygen use in muscle. Iron is part of oxygen-handling proteins.

What the name includes. Dietary iron comes as heme and nonheme iron; plant and fortified foods supply nonheme, while animal foods can supply both.

Where it appears. Meat, seafood, beans, lentils, nuts and iron-fortified grain products provide iron in different amounts and chemical forms.

How the body handles it. Transferrin carries iron through blood; ferritin provides storage, while hepcidin helps regulate how much iron enters the circulation.

Reference for Iron

Zinc Essential mineral

Participates in many enzymes, DNA and protein synthesis, cell division and wound repair. A catalytic and structural participant in cell chemistry.

What the name includes. Zinc, symbol Zn, is the mineral element incorporated into zinc-containing enzymes and structural proteins throughout the body.

Where it appears. Meat, seafood and dairy supply zinc, as do beans, nuts and whole grains, although the amount absorbed differs.

How the body handles it. Phytate in some plant foods can bind zinc and reduce absorption; intestinal uptake and losses help regulate the body's zinc balance.

Reference for Zinc

Copper Essential mineral

Copper-containing enzymes help energy production, iron handling and connective-tissue synthesis. Ceruloplasmin is a copper-containing protein involved in iron metabolism.

What the name includes. Copper, symbol Cu, is the trace element incorporated into cuproenzymes, the copper-containing proteins that carry out particular chemical reactions.

Where it appears. Shellfish, nuts, seeds, whole grains and chocolate are examples of copper sources, along with organ meats such as liver.

How the body handles it. Copper is absorbed in the upper small intestine; the liver helps regulate it by sending excess copper into bile.

Reference for Copper

Iodine Essential mineral

A constituent of thyroid hormones, which help regulate metabolic activity and development. Thyroxine (T4) and triiodothyronine (T3) contain iodine.

What the name includes. Iodine occurs in food as iodide, iodate and other forms; iodate is converted to iodide before absorption.

Where it appears. Iodized salt, fish, other seafood, eggs and dairy can supply iodine, while amounts in seaweed vary widely.

How the body handles it. Absorbed iodide circulates to the thyroid, which concentrates it to make thyroid hormones; much of the remainder leaves in urine.

Reference for Iodine

Selenium Essential mineral

Selenoproteins participate in thyroid-hormone metabolism and protection from oxidative damage. Glutathione peroxidases are selenium-containing enzymes.

What the name includes. Food selenium is commonly incorporated into the amino acids selenomethionine and selenocysteine, rather than present as a separate free element.

Where it appears. Seafood, meat, eggs and grains provide selenium; Brazil nuts can be especially rich, and plant-food content varies with soil.

How the body handles it. The body processes absorbed selenium forms into intermediates used to build selenoproteins, while urinary excretion helps regulate selenium balance.

Reference for Selenium

Manganese Essential mineral

A cofactor in enzymes handling nutrients, reactive oxygen species and tissue formation. Manganese superoxide dismutase and arginase use manganese.

What the name includes. Manganese, symbol Mn, is a trace mineral used by selected enzymes; magnesium, symbol Mg, is a different mineral.

Where it appears. Whole grains, nuts, legumes, leafy vegetables and tea are examples of foods and drinks that contribute manganese.

How the body handles it. After intestinal uptake, manganese travels bound to blood proteins; the body regulates its balance largely through absorption and biliary excretion.

Reference for Manganese

Molybdenum Essential mineral

Part of a cofactor required by enzymes that process sulfur-containing compounds and other molecules. Sulfite oxidase and xanthine oxidase require a molybdenum cofactor.

What the name includes. Molybdenum is a trace element assembled into the molybdenum cofactor, a reusable helper for a small group of enzymes.

Where it appears. Beans, lentils and other legumes supply molybdenum, as do whole grains, nuts, milk and organ meats such as liver.

How the body handles it. After uptake from the digestive tract, molybdenum enters tissue cofactors; the kidneys regulate its levels mainly through urinary excretion.

Reference for Molybdenum

Indispensable amino acids

Nine amino acids require dietary supply. They are members of the larger amino-acid family, not the complete list of amino acids used by the body.

Histidine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. His (H) has the side group CH₂–imidazole. Its protonation depends strongly on pH and local environment; it is not always positively charged. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Histidine · Reference 2 for Histidine

Isoleucine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Ile (I) has the side group CH(CH₃)–CH₂–CH₃. A branched side chain with a second stereocenter. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Isoleucine · Reference 2 for Isoleucine

Leucine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Leu (L) has the side group CH₂–CH(CH₃)₂. A branched nonpolar side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Leucine · Reference 2 for Leucine

Lysine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Lys (K) has the side group (CH₂)₄–NH₃⁺. Usually positively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Lysine · Reference 2 for Lysine

Methionine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Met (M) has the side group CH₂–CH₂–S–CH₃. Contains sulfur in a thioether. This is different from cysteine’s thiol. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Methionine · Reference 2 for Methionine

Phenylalanine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Phe (F) has the side group CH₂–phenyl. An aromatic side chain that is largely hydrophobic. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Phenylalanine · Reference 2 for Phenylalanine

Threonine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Thr (T) has the side group CH(OH)–CH₃. Its side chain includes a hydroxyl group. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Threonine · Reference 2 for Threonine

Tryptophan Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Trp (W) has the side group CH₂–indole. A bulky aromatic side chain, usually classed as largely nonpolar. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Tryptophan · Reference 2 for Tryptophan

Valine Indispensable amino acid

An indispensable amino acid used in proteins.

What the name includes. Val (V) has the side group CH(CH₃)₂. A branched nonpolar side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Valine · Reference 2 for Valine

Essential fatty acids

LA and ALA are two specific polyunsaturated fatty acids. Omega-6 and omega-3 are family names, not individual nutrients.

Linoleic acid Essential fatty acid

Linoleic acid is an omega-6 polyunsaturated fatty acid that humans must obtain from the diet.

What the name includes. LA is 18:2 omega-6, with two cis double bonds. It is both polyunsaturated and omega-6; the two labels answer different questions.

Where it appears. Many vegetable oils, nuts and seeds provide linoleic acid as part of mixed dietary lipids.

How the body handles it. LA contributes to membrane lipids and can supply material for longer omega-6 fatty acids. Humans cannot synthesize linoleic acid, so dietary supply is needed.

Reference for Linoleic acid

Alpha-linolenic acid Essential fatty acid

Alpha-linolenic acid is an omega-3 polyunsaturated fatty acid that humans must obtain from the diet. EPA and DHA are other omega-3s. Conversion from ALA is limited; they are not additional universally indispensable adult fatty acids.

What the name includes. ALA is 18:3 omega-3, with three cis double bonds. EPA (20:5) and DHA (22:6) are longer members of the omega-3 family.

Where it appears. Flax, chia, walnuts and some vegetable oils provide ALA. Fish and algae can supply the longer omega-3s EPA and DHA.

How the body handles it. ALA is a dietary essential. Conversion to EPA and DHA is limited, so sharing an omega family does not make these fatty acids interchangeable.

Reference for Alpha-linolenic acid

Carbohydrate families

Different structures and digestive routes. Family membership is a chemical description, not a health rating.

Sugars Carbohydrate family

Sugars include single units (monosaccharides) and pairs (disaccharides). The adjacent tiles show familiar examples. Glucose is an important body fuel, but the body can also make it; an individual sugar is not classified like an indispensable amino acid.

What the name includes. Monosaccharides are single sugars; disaccharides join two. These are structural categories. A food can contain both, alongside starch, fiber, fat and protein.

Where it appears. Fruit, milk and many other foods contain sugars. Sucrose is the sugar commonly used at the table; lactose occurs in milk.

How the body handles it. Disaccharides must be split before absorption. Glucose, galactose and fructose use different transport and processing routes; the liver helps handle the arriving supply.

Reference 1 for Sugars · Reference 2 for Sugars · Reference 3 for Sugars

Starch Carbohydrate family

Amylose is mostly unbranched; amylopectin is branched. Digestible starch supplies glucose. Resistant starch escapes small-intestinal digestion and overlaps the fiber category.

What the name includes. Amylose is mostly unbranched; amylopectin is branched. Both contain glucose, but their connections and arrangement influence how the material behaves.

Where it appears. Rice, potatoes, bread, beans and lentils can supply starch. Cooking, cooling and the surrounding food structure can alter its digestibility.

How the body handles it. Digestible starch supplies glucose. Resistant starch escapes some small-intestinal digestion and can reach the colon, where microbes may ferment it.

Reference 1 for Starch · Reference 2 for Starch · Reference 3 for Starch · Reference 4 for Starch

Dietary fiber Functional food category

Dietary fiber includes nondigestible carbohydrates and lignin. Some fibers are fermented by microbes; their effects depend on structure and properties. Cellulose, pectins, oat/barley beta-glucans, inulin/fructans, resistant starch and psyllium show different combinations of properties. Lignin is not a carbohydrate. Fiber has dietary guidance and important physiological roles; a family badge does not mean it is unnecessary. Fiber is not simply the biggest carbohydrate chain: digestibility matters, and some fibers are shorter oligosaccharides. Starch can also form very long chains.

What the name includes. Solubility asks how fiber behaves in water. Viscosity asks whether it thickens. Fermentability asks whether microbes can break it down. These are overlapping properties.

Where it appears. Oats, beans, vegetables, fruit, whole grains, nuts and seeds provide different fiber mixtures. No single food stands for every fiber type.

How the body handles it. Fiber resists digestion in the small intestine. Microbes ferment some into short-chain fatty acids; the unfermented fraction and microbial biomass contribute to stool.

Reference 1 for Dietary fiber · Reference 2 for Dietary fiber · Reference 3 for Dietary fiber

Protein family

The food family that supplies amino acids for many structures and working molecules.

Proteins Macronutrient family

Proteins are amino-acid chains folded into working structures. The standard protein alphabet has twenty amino acids: nine indispensable in adult diets and eleven the body can usually make. A protein need not contain all twenty types. Its sequence, length and folding matter. The body needs an adequate overall supply of amino acids and nitrogen, including the nine indispensable amino acids.

What the name includes. Proteins are ordered amino-acid chains. Chain length, sequence, folding, processing and assembly all matter; a short list of ingredients does not specify a working protein.

Where it appears. Beans, lentils, soy, dairy, eggs, fish, meat, nuts and seeds supply protein. Foods differ in amino-acid proportions and digestibility.

How the body handles it. Digestion supplies amino acids and small peptides. Absorbed and recycled amino acids join a shared supply for synthesis and other metabolism, with no dedicated surplus-protein tank.

Reference 1 for Proteins · Reference 2 for Proteins · Reference 3 for Proteins · Reference 4 for Proteins

Lipid families & structures

Storage structures, membranes, saturation and geometry describe different features and can overlap.

Triglycerides Storage-lipid family

Most dietary fat and stored body fat is triglyceride: a glycerol backbone joined to three fatty-acid chains. The three chains need not be identical.

What the name includes. Triacylglycerol and triglyceride name the same structure: glycerol attached to three fatty acids. Mono- and diacylglycerols have one and two fatty-acyl tails.

Where it appears. Oils, butter, nuts, seeds and many animal foods contain triglycerides with mixtures of fatty-acid tails.

How the body handles it. Digestion releases fatty acids and monoacylglycerols. Intestinal cells rebuild much of the long-chain triglyceride and package it in chylomicrons for lymph, then blood.

Reference 1 for Triglycerides · Reference 2 for Triglycerides · Reference 3 for Triglycerides

Saturated fatty acids Fatty-acid family

Saturated fatty-acid chains have no carbon-carbon double bonds. Palmitic and stearic acids. Saturation describes bond structure; it does not summarize the nutritional profile of a whole food.

What the name includes. Saturation means no carbon–carbon double bonds in a fatty-acid chain. It does not mean the chain has no flexibility, or that an entire food contains only one fatty acid.

Where it appears. Butter, coconut oil, meat and dairy fat contain saturated fatty acids in differing proportions. Foods also contain other fatty-acid types.

How the body handles it. After digestion and transport, fatty acids can be oxidized, stored or used in other lipids. Saturation is a structural feature, not a destination label.

Reference for Saturated fatty acids

Monounsaturated fatty acids Fatty-acid family

A monounsaturated fatty-acid chain has one carbon-carbon double bond. Oleic acid.

What the name includes. A monounsaturated fatty acid has one carbon–carbon double bond. Geometry and omega position add information; oleic acid is a cis omega-9 example.

Where it appears. Olive oil, avocados and many nuts contain monounsaturated fatty acids alongside other kinds.

How the body handles it. These fatty acids can enter fuel pathways or become parts of storage and membrane lipids. A shared bond count does not make every monounsaturated molecule identical.

Reference for Monounsaturated fatty acids

Polyunsaturated fatty acids Contains essential members

Polyunsaturated fatty-acid chains have two or more carbon-carbon double bonds. LA and ALA belong to this group. Omega numbering locates the first double bond from the methyl end. It is a second classification, alongside saturation.

What the name includes. Polyunsaturated means at least two carbon–carbon double bonds. Omega-3 and omega-6 specify the first double bond from the methyl end, so a fatty acid can be both polyunsaturated and omega-3, or both polyunsaturated and omega-6.

Where it appears. Seeds, nuts, vegetable oils and fish provide different polyunsaturated fatty acids. The specific mix matters to the name, even within one omega family.

How the body handles it. Polyunsaturated fatty acids contribute to membranes, storage and signaling precursors as well as energy metabolism. LA and ALA are dietary essentials.

Reference 1 for Polyunsaturated fatty acids · Reference 2 for Polyunsaturated fatty acids

Trans fatty acids Double-bond geometry

Trans describes the arrangement around a carbon-carbon double bond in an unsaturated fatty acid. Trans is a geometry label, not a fourth double-bond-count category. It can overlap mono- and polyunsaturated structures.

What the name includes. Trans geometry places groups differently around a double bond. A trans fatty acid still has that double bond and therefore remains unsaturated.

Where it appears. Some trans fats arise in ruminant foods; industrial partial hydrogenation can also produce them. These are origin distinctions, not different definitions of trans.

How the body handles it. Trans and cis chains can share a formula and double-bond count while differing in geometry. The builder compares oleic and elaidic acid to isolate this structural change.

Reference for Trans fatty acids

Phospholipids Membrane-lipid family

Phospholipids are phosphate-containing lipids. Their water-interacting and water-avoiding regions help organize cell membranes. Phosphatidylcholine is one member. Choline is an essential nutrient used to make particular phospholipids; the whole lipid family is not one extra essential nutrient.

What the name includes. A phosphate-bearing region interacts with water while other regions avoid it. Glycerophospholipids and sphingomyelin have different backbones; the builder uses one glycerol-based example.

Where it appears. Eggs, soybeans and cell-containing foods supply phospholipids. The body also synthesizes them.

How the body handles it. Phospholipids help form cell membranes and lipoprotein surfaces. Their components are digested, absorbed and reused; a dietary membrane does not simply become an intact body membrane.

Reference 1 for Phospholipids · Reference 2 for Phospholipids · Reference 3 for Phospholipids

Sterols Ring-structured lipid family

Cholesterol is a sterol used in cells and as a precursor to steroid hormones, bile acids and vitamin D. The body makes cholesterol. Having an important body function does not make dietary cholesterol essential.

What the name includes. Sterols have a ring-based structure. Cholesterol is the principal animal sterol; phytosterols are related plant molecules. Neither is a three-tailed triglyceride.

Where it appears. Animal foods can supply cholesterol; plant foods supply phytosterols. Humans also synthesize cholesterol.

How the body handles it. Cholesterol contributes to membranes, bile acids and steroid molecules. Lipoproteins carry it through blood; LDL and HDL describe particles, not separate cholesterol chemicals.

Reference 1 for Sterols · Reference 2 for Sterols

Cis fatty acids Double-bond geometry

Cis describes the arrangement around a carbon-carbon double bond in an unsaturated fatty acid. The carbon-chain segments lie on the same side of that bond. Cis and trans describe geometry. Mono- and polyunsaturated describe how many double bonds there are; these classifications overlap.

What the name includes. Cis describes geometry around a double bond. Most naturally occurring unsaturated fatty acids have cis double bonds; a chain can have more than one.

Where it appears. Oleic acid in olive oil and linoleic acid in many seed oils are cis examples with different double-bond counts.

How the body handles it. The geometry influences a chain’s shape and packing. It is separate from chain length, saturation count and the omega family.

Reference for Cis fatty acids

Selected plant-compound families

A few useful examples, not a complete catalog. These labels can overlap nutrient chemistry and do not establish a health benefit for every member.

Polyphenols Selected phytochemical family: A broad group of plant-associated compounds that includes flavonoids, stilbenes and lignans. Flavonoids include anthocyanin pigments. Polyphenols are not one essential nutrient. Chemical activity in a laboratory does not establish a dietary health benefit. Reference 1 for Polyphenols · Reference 2 for Polyphenols
Carotenoids Some supply vitamin A: Some carotenoids can be converted into vitamin A; others cannot. Only provitamin A carotenoids contribute to vitamin A supply. Beta-carotene can supply vitamin A. Lycopene, lutein and zeaxanthin cannot. Reference for Carotenoids
Glucosinolates Selected phytochemical family: Preparation, chewing and digestion can break glucosinolates into compounds such as indoles and isothiocyanates. Found in plants such as broccoli and cabbage. This chemistry is not proof that a particular food or isolated compound prevents cancer. Reference for Glucosinolates
Allium sulfur compounds Selected phytochemical group: Garlic contains sulfur compounds whose forms change when its tissue is cut or crushed. Allicin is one familiar example. This is a food-source grouping, not every organosulfur compound. Glucosinolates are also sulfur-containing compounds. Reference for Allium sulfur compounds
Phytosterols Plant-sterol family: Phytosterols are plant sterols found in plant-cell structures and plant foods. They overlap the sterol family. Their presence in food does not make them an established essential human nutrient. Reference for Phytosterols

Mineral classification details

Required elements, useful exposures and disputed essentiality need different labels.

Sulfur Required within other nutrients: Sulfur occurs in methionine and cysteine. Their breakdown supplies sulfate for building sulfated compounds. There is no separate dietary sulfate requirement when sulfur-amino-acid needs are met. Sulfur is often listed as a major mineral. Methionine and cysteine supply much of the sulfate used in metabolism; this is not a separate elemental sulfur target. Reference for Sulfur
Cobalt Required within vitamin B12: Cobalt is a constituent of vitamin B12. Its essential nutritional role is covered by the requirement for that vitamin. See the vitamin B12 card. Cobalt in vitamin B12 is not interchangeable with consuming elemental cobalt or other cobalt compounds. Reference for Cobalt
Fluoride Beneficial; not essential: Fluoride helps prevent tooth decay. This benefit is distinct from being essential for human growth or having a recognized deficiency disease. It can appear in nutrient reference tables even though essentiality has not been established. A nutrient can have an Adequate Intake reference value for a benefit without being classified as essential. Reference 1 for Fluoride · Reference 2 for Fluoride
Chromium Essentiality disputed: The US Food and Nutrition Board classified chromium as essential in 2001 and has not re-evaluated it. EFSA found no convincing evidence of an essential human requirement. The US classification dates to 2001; EFSA reassessed essentiality in 2014. Essentiality remains unsettled despite its appearance in some nutrient-reference tables. Reference for Chromium

Synthesis & conditional needs

Some important molecules are synthesized by the body. Conditional dietary need depends on the molecule and the physiological setting.

Conditionally essential amino acids Depends on physiological context

Ordinarily synthesizable amino acids can require a dietary source in particular physiological or illness states. Examples include arginine, cysteine, glutamine and tyrosine. This is not a fixed extra requirement for every healthy adult, or a label for everyday psychological stress. Reference for Conditionally essential amino acids

Carnitine Usually made; conditional need

Carnitine helps transport long-chain fatty acids into mitochondria. Healthy adults generally synthesize enough. Dietary need can become important in specific conditions. A role in fatty-acid transport does not by itself establish a fat-loss benefit. Reference for Carnitine

ATP Body-made energy carrier

ATP helps couple energy-releasing reactions to cellular work. Cells continually use and regenerate it. ATP is central to metabolism, but it is not an essential dietary nutrient or a long-term fuel store. Reference for ATP

Glycogen Body-made glucose store

The body builds glycogen from glucose, especially in liver and muscle. Liver glycogen can support blood glucose; muscle glycogen mainly supports that muscle. Glycogen and plant starch are related storage carbohydrates, but they are not the same structure or dietary requirement.

What the name includes. Glycogen is a highly branched glucose polymer. Most connections are α(1→4), with α(1→6) bonds at branch points.

Where it appears. Glycogen is shown here as a body-made glucose store, not a separate essential food requirement.

How the body handles it. Liver glycogen helps support blood glucose. Muscle glycogen is a local reserve for that muscle’s work; the two stores have different jobs.

Reference 1 for Glycogen · Reference 2 for Glycogen

Creatine Body-made; also found in food

The body makes creatine from amino-acid precursors. Phosphocreatine helps regenerate ATP, particularly during brief, intense demands. Creatine is not one of the nine indispensable amino acids. Creatinine is a different molecule: a breakdown product excreted in urine. Reference for Creatine

Monosaccharides · single sugars

Selected single sugar units. These are examples within the sugar family, not three extra dietary essentials.

Glucose Monosaccharide · one sugar unit

Glucose is a single sugar. Starch and glycogen are built from glucose units.

What the name includes. One sugar unit. Starch digestion supplies glucose, and glucose is also present in foods such as fruit. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

Where it appears. Glucose occurs in fruits and honey and is released when digestible starch is broken down.

How the body handles it. Absorbed glucose reaches portal blood and the liver, then other tissues. It can support ATP regeneration, glycogen storage and synthesis.

Reference 1 for Glucose · Reference 2 for Glucose

Fructose Monosaccharide · one sugar unit

Fructose is a single sugar. It has the same molecular formula as glucose but a different arrangement of atoms.

What the name includes. One sugar unit with a different arrangement from glucose. Glucose and fructose are the two units in sucrose. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

Where it appears. Fruit and honey contain fructose. Sucrose also provides fructose when its bond is split.

How the body handles it. This single sugar can be absorbed without first splitting a sugar–sugar bond. It reaches portal blood; intestinal and liver processing help integrate its carbon into metabolism.

Reference 1 for Fructose · Reference 2 for Fructose

Galactose Monosaccharide · one sugar unit

Galactose is a single sugar. Joined to glucose, it forms lactose.

What the name includes. One sugar unit. In lactose, galactose is linked to glucose; digestion separates them. Glucose, fructose and galactose share the formula C₆H₁₂O₆ but arrange atoms differently.

Where it appears. Much dietary galactose comes from digestion of lactose in milk and milk products.

How the body handles it. This single sugar can be absorbed without first splitting a sugar–sugar bond. It reaches portal blood; intestinal and liver processing help integrate its carbon into metabolism.

Reference 1 for Galactose · Reference 2 for Galactose

Disaccharides · paired sugars

Two sugar units joined by a glycosidic bond. Digestion separates them before absorption.

Sucrose Disaccharide · two sugar units

Sucrose joins two single sugars: glucose and fructose.

What the name includes. Glucose + Fructose form this two-sugar combination. α(1↔2)β describes the connecting bond.

Where it appears. Sucrose occurs in many plants and is the sugar commonly used at the table.

How the body handles it. Enzymes at the small-intestinal surface split this disaccharide into single sugars before absorption. The products can then enter portal blood.

Reference 1 for Sucrose · Reference 2 for Sucrose

Lactose Disaccharide · two sugar units

Lactose joins two single sugars: glucose and galactose.

What the name includes. Galactose + Glucose form this two-sugar combination. β(1→4) describes the connecting bond.

Where it appears. Lactose is the characteristic sugar in milk; amounts differ across milk products.

How the body handles it. Lactase splits lactose at the intestinal surface. Lower lactase activity leaves more lactose available to colonic microbes; digestion depends on the person and the amount.

Reference 1 for Lactose · Reference 2 for Lactose

Maltose Disaccharide · two sugar units

Maltose joins two single sugars: glucose and glucose.

What the name includes. Glucose + Glucose form this two-sugar combination. α(1→4) describes the connecting bond.

Where it appears. Maltose occurs during starch breakdown, including in germinated grains and malted foods.

How the body handles it. Enzymes at the small-intestinal surface split this disaccharide into single sugars before absorption. The products can then enter portal blood.

Reference 1 for Maltose · Reference 2 for Maltose

Other standard amino acids

The other eleven of the twenty standard protein amino acids. Adults can usually synthesize these; a dietary need can arise for some in particular physiological settings. “Nonessential” does not mean unused or unimportant.

Alanine Standard amino acid · usually synthesized

Alanine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Ala (A) has the side group CH₃. A small nonpolar methyl side chain. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Alanine · Reference 2 for Alanine

Arginine Standard amino acid · usually synthesized

Arginine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Arg (R) has the side group (CH₂)₃–guanidinium. Usually positively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Arginine · Reference 2 for Arginine

Asparagine Standard amino acid · usually synthesized

Asparagine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Asn (N) has the side group CH₂–CONH₂. An uncharged amide side chain in the usual physiological context. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Asparagine · Reference 2 for Asparagine

Aspartic acid Standard amino acid · usually synthesized

Aspartic acid is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Asp (D) has the side group CH₂–COO⁻. Usually negatively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Aspartic acid · Reference 2 for Aspartic acid

Cysteine Standard amino acid · usually synthesized

Cysteine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Cys (C) has the side group CH₂–SH. Its thiol can form a disulfide bond with another cysteine under suitable conditions. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Cysteine · Reference 2 for Cysteine

Glutamic acid Standard amino acid · usually synthesized

Glutamic acid is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Glu (E) has the side group CH₂–CH₂–COO⁻. Usually negatively charged near physiological pH. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Glutamic acid · Reference 2 for Glutamic acid

Glutamine Standard amino acid · usually synthesized

Glutamine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Gln (Q) has the side group CH₂–CH₂–CONH₂. An amide side chain with one more methylene group than asparagine. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Glutamine · Reference 2 for Glutamine

Glycine Standard amino acid · usually synthesized

Glycine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Gly (G) has the side group H. The smallest side group. Glycine has no chiral alpha carbon and gives the backbone unusual flexibility. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Glycine · Reference 2 for Glycine

Proline Standard amino acid · usually synthesized

Proline is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Pro (P) has the side group Ring to backbone N. The side chain reconnects to the backbone nitrogen, restricting its geometry. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Proline · Reference 2 for Proline

Serine Standard amino acid · usually synthesized

Serine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Ser (S) has the side group CH₂–OH. Its hydroxyl group can participate in hydrogen bonding. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Serine · Reference 2 for Serine

Tyrosine Standard amino acid · usually synthesized

Tyrosine is one of the twenty standard protein amino acids. Usually synthesized by the body. See the conditional-needs entry for the nutritional qualification.

What the name includes. Tyr (Y) has the side group CH₂–phenyl–OH. An aromatic side chain with a polar hydroxyl group. Categories describe tendencies, not absolute behavior. Side-group chemistry is a separate classification from whether dietary supply is indispensable.

Where it appears. This amino acid occurs within food proteins, alongside other amino acids. Protein-rich foods supply mixtures; no single food belongs exclusively to one amino acid.

How the body handles it. Absorbed and recycled amino acids can be assembled into new proteins according to cellular instructions. Their side groups influence interactions and folding; a particular mixture alone does not specify a protein.

Reference 1 for Tyrosine · Reference 2 for Tyrosine

Original chart by Shreyam Adhikari / Carbon Atlas. Learning framework inspired by Dr. Andy Galpin; classifications refined from the references in each tile.

03 / THE WHOLE JOURNEY

Energy changes form.
Matter goes somewhere.

A meal brings two things into the body: material to rearrange, and chemical energy to make useful work possible.

Metabolism is the whole network of reactions that does both. Catabolism breaks molecules down. Anabolism builds molecules up. Your body does these at the same time: renewing a protein while spending energy to keep a cell alive.

A calorie measures energy. ATP is a molecule cells repeatedly regenerate to help drive work—muscle contraction, ion pumping, and the assembly of other molecules. Fuel molecules hold much larger reserves than the small working supply of ATP. [3][4]

Atoms do not become energy and vanish. During oxidation, their arrangement changes; energy is transferred through coupled reactions, and some is released as heat. Keep these two stories separate as you follow the map.

The same illustrated mixed meal contains several nutrient families. — ONE MEAL · DIFFERENT JOURNEYS
What changes along the way?
Choose a component. Follow its changing form, its transport route, and what it can become.

A sourced atomic model of glucose, one building block of starch. — ONE MEAL · DIFFERENT JOURNEYS
What changes along the way?
Choose a component. Follow its changing form, its transport route, and what it can become.

04 / DIGESTION & ABSORPTION

A meal enters.
Different routes open.

Eating puts food inside the digestive tract. Absorption is the next crossing: from the intestine into the body’s transport systems.

Digestion changes the package before the journey continues. The liver makes bile, the gallbladder stores and releases it, and the pancreas supplies enzymes and bicarbonate to the small intestine. Bile helps disperse fat; enzymes do the bond-breaking. [5][6]

ALONG THE DIGESTIVE TRACT

Break down. Then cross the wall.

01
Mouth
Chewing makes smaller pieces. Salivary amylase begins starch digestion.
02
Stomach
Mixing, acid and pepsin begin substantial protein digestion. The meal leaves gradually.
03
Small intestine
Pancreatic enzymes and enzymes at the intestinal surface finish much of digestion. Bile disperses fat. Most nutrient absorption happens here.
04
Colon
Microbes ferment some remaining fiber. Water and electrolytes are absorbed; unabsorbed material leaves in stool.

A simplified route, not an anatomical drawing or a map of digestion speed. [5]

AFTER ABSORPTION

Does everything go to the liver first?

Sugars and amino acids largely take portal blood to the liver. Most long-chain dietary fat takes lymph to the wider circulation first.

Sugars

Glucose, fructose, galactose

Small intestine

Intestinal blood
Portal vein
Liver
Wider circulation

The liver processes part of this supply; glucose also passes on to other tissues. Fructose and galactose enter different processing steps before joining shared pathways. [6][9]

Protein

Amino acids after peptide breakdown

Small intestine

Intestinal blood
Portal vein
Liver & other tissues

Intestinal cells use some amino acids themselves. The liver handles much of the arriving supply; others, including much of the branched-chain amino-acid supply, reach tissues such as muscle. [6][22][73]

Most dietary fat

Long-chain fatty acids + monoacylglycerols

Small intestine

Rebuilt triglycerides
Chylomicrons
Lymph → blood
Muscle & fat tissue

Intestinal cells repackage these fats. Lipoprotein lipase releases fatty acids for tissues; the remaining chylomicron particles are later taken up by the liver. This route does not visit the liver first. [6][17]

Fat-soluble vitamins

Vitamins A, D, E and K

Small intestine

Absorbed with lipids
Chylomicrons
Lymph → blood

They travel with the dietary lipid route initially, then have vitamin-specific transport and storage. These are helpers and materials, not ATP-producing fuels. [6]

Water-soluble vitamins & minerals

Individual vitamins and mineral ions

Mainly small intestine; sites vary

Intestinal absorption
Portal blood
Liver & tissues

There is no single vitamin or mineral transporter. B12 needs intrinsic factor and is absorbed in the terminal ileum. Iron is mainly absorbed near the start of the small intestine. Calcium absorption occurs along it, with mechanisms that vary by region. [6][30][75][77]

Fiber & water

Microbial products; water stays water

Small intestine and colon

Some fiber → microbes
Short-chain fatty acids
Colon cells & portal blood

Butyrate is important fuel for colon cells; other microbial products reach the liver and circulation. Water needs no digestion and is absorbed mostly in the small intestine, with more recovered in the colon. [5][8]

Useful exceptions: shorter fats, choline and plant compounds

Short-chain and many medium-chain fatty acids can enter portal blood more directly; chain length and the meal affect packaging. This is why the main diagram says “most dietary fat.” [76]

Choline’s route depends on its chemical form: water-soluble forms commonly reach portal blood, while lipid-associated forms can accompany lipid transport. Plant compounds vary even more; some are absorbed and modified by intestinal cells and the liver, while others are transformed by microbes or leave unabsorbed. They do not all share one route. [37][78]

05 / CARBOHYDRATE'S ROUTE

Sugar, starch, fiber.
One family. Different paths.

FROM THE INTESTINE TO USE OR STORAGE

Absorbed sugars
Portal blood → liver
Glucose in blood
Cells / glycogen

After a starchy meal, absorbed glucose joins the blood supply. Cells can use it to regenerate ATP or to make other molecules. The liver handles much of the absorbed fructose and galactose through different chemical steps. Fiber follows the microbial branch instead.

Glucose stored as glycogen has two main homes: the liver can release glucose to support the blood supply; muscle keeps its glycogen for its own work. Carbon can also enter fat synthesis, but there is no universal ‘glycogen full, now make fat’ switch. Use, synthesis and storage run together.

[8][9][19][20]

“Carbohydrate” describes a family of molecules, not a single food or a single metabolic destiny.

Monosaccharides are individual sugars: glucose, fructose and galactose. Disaccharides join two sugars: sucrose is glucose plus fructose; lactose is glucose plus galactose. Starch contains long glucose chains, including amylose and branched amylopectin. [7]

Simple and complex describe structure. They do not reliably predict the blood-glucose response to a whole meal. Cooking, processing, the food’s physical structure and the other ingredients all matter. [56]

TRY IT · 01

One sugar. A pair. A chain.

Start small. Choose an example and see how its building blocks connect.

MONOSACCHARIDE · ONE MOLECULE

GlucoseC₆H₁₂O₆

One sugar unit. Starch digestion supplies glucose, and glucose is also present in foods such as fruit.

Choose a single sugar

A sugar used throughout the body. All three examples share a formula, with different atom arrangements.

Blocks represent whole sugar units, not individual atoms. [7]

Why do the connections matter?

Monosaccharides are single sugars; disaccharides contain two units; polysaccharides contain many. Glucose and fructose make sucrose; galactose and glucose make lactose. Maltose and cellobiose both contain two glucose residues, but use different linkages. [7][70]

Joining two C₆H₁₂O₆ sugars gives C₁₂H₂₂O₁₁ + H₂O in the net condensation equation. This is bookkeeping, not a synthesis recipe: cells use appropriate enzymes and often activated sugar donors. Hydrolysis uses water to split the bond.

Glc means a glucose residue. Polymer drawings are short excerpts with schematic branch spacing. Choosing a different example compares structures; it does not show one polymer directly turning into another.

See a real glucose moleculeRotate and inspect the atoms in 3D

α-D-glucopyranose: a ball-and-stick molecular structure. A glucose ring, shown as alpha-D-glucopyranose. The ring has five carbons and one oxygen; the sixth carbon sits outside it. — A glucose ring, shown as alpha-D-glucopyranose. The ring has five carbons and one oxygen; the sixth carbon sits outside it. Computed ideal coordinates, not a measured shape in food. SDF bond orders are preserved; hydrogens start hidden. RCSB PDB: GLC · Structure data · CC0 data · 3Dmol.js (BSD license).

01 / USE

Spend it now.

Glucose can enter glycolysis, then connect to oxidative metabolism. It also supplies intermediates for building other molecules.

02 / STORE

Keep it as glycogen.

Liver glycogen helps maintain blood glucose. Muscle glycogen supports the muscle’s own work. They are different stores with different jobs. [9]

03 / BUILD

Make something new.

Carbohydrate carbon can also contribute to new fatty acids through de novo lipogenesis. This is a separate synthesis pathway.

Go deeper: how carbohydrate carbon can become fat

THE STORAGE BRANCH

How carbohydrate can become fat.

Glucose
Pyruvate
Mitochondrial acetyl-CoA
Citrate export
Cytosolic acetyl-CoA
Fatty-acid synthesis

Citrate carries carbon out of mitochondria. Cytosolic acetyl-CoA can become malonyl-CoA; fatty acid synthase builds a chain using ATP-supported chemistry and NADPH. Fatty acids can then be joined to glycerol to make triglyceride. [51][52]

Does glycogen have to be completely full first?

There is no single universal overflow switch. Several pathways run together, with rates that depend on conditions. Carbohydrate can influence fat storage both through new fat synthesis and through increased carbohydrate oxidation that reduces fat oxidation. Controlled feeding studies show different contributions under different diets. [19][20]

Fiber takes a microbial detour.

Fiber resists digestion in the small intestine; chain length alone does not define it. Gut microbes can use some of it, producing short-chain fatty acids such as acetate, propionate and butyrate that can enter host metabolism. But fibers differ in how they interact with water and microbes. [8][65]

A WORLD WITHIN THE FAMILY

One word. Three different questions.

Soluble does not automatically mean viscous, and insoluble does not mean unfermentable. These properties overlap. [64][65]

Soluble / insoluble: How readily a fiber dissolves in water.
Viscous / nonviscous: Whether it thickens the surrounding fluid under the conditions present.
Fermentability: How readily gut microbes break it down. This is a spectrum.

Conceptual amber fiber strands beside separate sage and charcoal microbial forms. Some strands continue past them. — **Some fiber meets a microbial pathway.** Microbes can ferment it into short-chain fatty acids and gases. Unfermented material contributes to stool; microbial biomass does too. [8][68]Conceptual artwork. Strands and microbes are symbols, not molecular structures or a measured community. Fiber does not turn directly into bacteria.

EXPLORE SIX EXAMPLES

Inulin / fructans

Examples occur in chicory and onions.

In water: Generally soluble
Thickening: Usually low viscosity
Microbial use: Readily fermented

This is the useful counterexample: a soluble fiber can feed microbial fermentation without forming a thick gel.

[64][65][67]

Typical properties, not fixed scores. Fiber form, chain size, food structure, processing and the gut community can change behavior. Whole foods contain mixtures; these examples do not predict an individual response.

Compare all six fibers together

Typical properties, with qualifications. These are descriptions, not health rankings.
Fiber example	In water	Thickening	Microbial use
Cellulose	Insoluble	Nonviscous	Generally limited; variable
Pectins	Usually soluble	Can thicken or gel	Readily fermented
Oat / barley β-glucans	Soluble fraction varies	Can be viscous; depends on form	Fermentable
Inulin / fructans	Generally soluble	Usually low viscosity	Readily fermented
Resistant starch	Often insoluble (RS2 / RS3)	Usually low (RS2 / RS3)	Often substantial; varies by type
Psyllium husk	Soluble + insoluble fractions	Viscous; gel-forming	Relatively limited

[64][65][66][67] These examples are not an exhaustive inventory: hemicelluloses, gums and other fibers have their own varied properties. Lignin is included in dietary-fiber definitions but is not a carbohydrate.

06 / DIETARY FAT'S ROUTE

A compact store.
A different entrance.

DIETARY LIPIDS & LIVING FAT TISSUE

Food triglyceride
Digested & rebuilt
Chylomicron in blood
Fatty acids enter cells

Fat in food does not travel intact into a patch of body fat. Most dietary triglycerides are digested, absorbed and rebuilt before transport. Their fatty acids can still become stored body triglyceride. The chemistry is related; the food, transport particle and living tissue are different things.

Cells can oxidize fatty acids for ATP, store them, or incorporate them into lipids such as membrane phospholipids. Cholesterol helps build membranes and is a precursor for steroid hormones and bile acids. Fat does not replace the amino acids required to build protein.

[6][15][17][21]

A triglyceride is three fatty acids attached to glycerol. Its fatty-acid chains hold chemical fuel that can feed the same network used by carbohydrate.

In stored fat, lipolysis separates those pieces. A released fatty acid can be used, returned to storage, or sent to another tissue. Release is not the same as oxidation. [17]

TRY IT · 02

Name the molecule. Then its tails.

“Triglyceride” describes the whole molecule. “Saturated” and “omega-3” describe its fatty-acid chains.

ONE GLYCEROL · UP TO THREE TAILSTriacylglycerol

Three tails on glycerol make a triglyceride—the main storage form of fat.

What is attached to glycerol?

3fatty-acyl tails3fatty-acid ester links

One tail: monoacylglycerol. Two: diacylglycerol. Three: triacylglycerol, also called a triglyceride.

Attachment map, not atom-counting geometry. Removing a tail represents ester hydrolysis using water. [15][71]

Why can one fat have several names?

These labels describe different features. Saturation counts C=C bonds. Cis/trans describes their geometry. Omega counts to the first C=C bond from the methyl end. Linoleic acid is both polyunsaturated and omega-6; α-linolenic acid is both polyunsaturated and omega-3. A triglyceride can contain different types of tail. [15][16]

Stearic acid. 18 carbons and no carbon–carbon double bonds. Single bonds permit many conformations; the straight-looking icon is simplified.

Cholesterol is a sterol. LDL and HDL are lipoprotein particles carrying lipids, not saturation categories. [42]

Attached fatty acids are fatty-acyl residues. Each formal ester condensation removes H₂O; ester hydrolysis uses H₂O. This is a structural comparison, not the cell’s synthesis sequence. Phosphatidylcholine also has phosphate ester linkages, separate from its two fatty-acid ester links. Other phospholipids can have other head groups or backbones.

See a real triglycerideOne backbone and three tails · explore in 3D

Tripalmitin: a ball-and-stick molecular structure. One example of a triglyceride: glycerol linked to three palmitate chains. Food and body fat contain many other combinations. — One example of a triglyceride: glycerol linked to three palmitate chains. Food and body fat contain many other combinations. Computed ideal coordinates, not a measured shape in food. SDF bond orders are preserved; hydrogens start hidden. RCSB PDB: 4RF · Structure data · CC0 data · 3Dmol.js (BSD license).

FOUR QUESTIONS · DIFFERENT KINDS OF ANSWER

Which kind of “fat”?

A lipid molecule, a fatty-acid chain, a transport particle and a tissue are different levels of the story.

What is the whole structure?

Lipids are a broad family. A triglyceride, a phospholipid and cholesterol have different structures and jobs.

The molecule

Triglycerides

Three fatty acids are attached to glycerol. This is a major form of dietary fat and the main lipid stored in fat cells.

Connect it to the journey

Digestion breaks the package apart. Intestinal cells rebuild much of it for transport in chylomicrons.

[15][17][42]

LA, ALA, EPA and DHA: read the names together

The notation counts carbons : carbon–carbon double bonds.
Fatty acid	Chain	Family and context
LALinoleic acid	18:2	Omega-6 Dietary essential; found in many seeds, nuts and vegetable oils.
ALAAlpha-linolenic acid	18:3	Omega-3 Dietary essential; examples include flax, chia and walnuts.
EPAEicosapentaenoic acid	20:5	Omega-3 Present in fish and some algal sources; formed from ALA only to a limited extent.
DHADocosahexaenoic acid	22:6	Omega-3 An important membrane fatty acid, including in retina and brain; fish and algal sources can supply it.

ALA conversion to EPA and DHA is limited. A shared omega family does not make these molecules interchangeable. EPA and DHA sit outside the builder’s five 18-carbon examples. [16]

Go deeper: fat oxidation and cholesterol transport

From a fatty acid to acetyl-CoA.

Fatty acids are activated to fatty acyl-CoA. Long-chain fatty acids use the carnitine shuttle to reach the mitochondrial matrix. Beta-oxidation shortens the chain, usually two carbons at a time, making acetyl-CoA and reducing equivalents for energy transfer. The acetyl-CoA joins the Krebs cycle. [50]

Glycerol takes a separate route toward carbohydrate metabolism. Common even-chain fatty acids do not provide a net gain of glucose through the usual acetyl-CoA pathway in humans. Odd-chain fatty acids have a different entry through propionyl-CoA. [57]

Two fatty acids—linoleic acid and alpha-linolenic acid—are essential in the diet. EPA and DHA are other omega-3s; conversion from alpha-linolenic acid is limited. “Omega” labels describe molecular positions, not an automatic verdict on a food. [16]

And what about cholesterol transport?

Chylomicrons carry much of the dietary fat arriving from the intestine. The liver exports VLDL, which can be remodeled through IDL into LDL. HDL participates in cholesterol transport and exchange. These are lipoprotein particles with different routes—not a single chylomicron → LDL → HDL chain. [42]

07 / PROTEIN'S ROUTE

From amino acids
to working proteins.

BUILDING AND RECYCLING, WITH A NITROGEN EXIT

Dietary protein
Amino acids
New proteins / other molecules
Breakdown when needed

The body assembles absorbed and recycled amino acids into its own proteins: enzymes, muscle machinery, transporters and much more. A protein-rich meal does not become the same amount of new muscle. Synthesis and breakdown continue throughout the day.

There is no dedicated surplus-protein tank. When amino acids are broken down, nitrogen is handled largely through liver urea production and urinary excretion. Carbon skeletons can join fuel pathways or glucose synthesis, depending on the amino acid. Protein can contribute energy while carbohydrate and fat are still available.

[21][22]

Eating protein supplies material. Your cells decide what to build with it.

Nine amino acids need a dietary supply because the body cannot make enough of them. The others still matter: “nonessential” describes our ability to make them. Mixing amino acids provides ingredients; a cell needs an instruction to assemble a particular protein. [3][69]

Ribbon and atoms of the folded Trp-cage miniprotein, from an experimental NMR model. — FROM BUILDING BLOCKS TO BIOLOGY
The order. The fold. The work.
Follow the material through four connected ideas.

Trp-cage · TC5b: a folded ribbon and atomic sticks. Trp-cage is a designed 20-residue miniprotein. The viewer uses all deposited atoms in one solution-NMR model; it is a structure example, not a protein from this meal. — FROM BUILDING BLOCKS TO BIOLOGY
The order. The fold. The work.
Follow the material through four connected ideas.

What makes it fold?

For many proteins in water, water-avoiding side groups cluster inside. Hydrogen bonds and other interactions help stabilize the structure. The sequence matters, and so do the surroundings. A working protein can still move and change shape. [82]

What if the shape changes?

Heat or changes in acidity can disrupt a protein’s usual structure: denaturation. Its activity may change or disappear. Unfolding a chain and cutting its peptide bonds are different changes; digestive enzymes do the cutting. [21][6]

FOUR NAMES YOU MAY COME ACROSS [21][82]

Primary: The amino-acid order.
Secondary: Local patterns, including helices and sheets.
Tertiary: The overall 3D arrangement of one chain.
Quaternary: Several chains assembled together, when present.

TRY IT · 03

Change a letter. Change the sequence.

Each tile is an amino-acid unit. Each connector is a peptide bond. Tap a tile to choose a position.

READ FROM N TERMINUS TO C TERMINUSA tripeptide

N endG–A–GC end

3amino-acid units2peptide bonds2types1fragment

Three positions, two types. Replace the middle alanine and watch the sequence change.

Replace Ala at position 2

Explore all 20 types by side group

The palette below shows five examples. Grouping describes chemistry, not dietary importance; charge depends on context and pH.

Sequence model, not a folding prediction. Eight positions is a display limit. [21][69]

Why does the side group matter?

POSITION 2

Alanine

CH₃

A small nonpolar methyl side chain.

Before joining, the free amino acid has an amino group, a carboxyl group, hydrogen and its side group around Cα. This diagram omits charges and group hydrogens. For proline, the side group also connects to nitrogen. This is not the formula of an internal residue.

A peptide bond joins a carbonyl carbon to the next nitrogen: C(=O)–N. Replacing a tile edits the sequence model; it is not a depicted chemical reaction. Real cells use activated amino acids on tRNAs, not direct condensation of free amino acids. Hydrolysis and synthesis use different machinery. [21][69]

Twenty standard types form an alphabet: a protein does not need one of each. Sequence and environment influence folding; some regions remain disordered. This model predicts neither a fold nor a function.

Protein turnover, essential amino acids and nitrogen disposal

Your proteins are continually renewed. Their amino acids can be reused—or broken down, with nitrogen and carbon taking different routes.

This turnover supports repair and adaptation. Muscle is active tissue, not a separate, inert protein storage tank. Changes in muscle size depend on the balance of synthesis and breakdown over time. [22][58]

Meet the nine essential amino acids

Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Each is required for protein synthesis; this list describes a nutritional category, not a supplement plan. [3]

WHEN AN AMINO ACID IS BROKEN DOWN

Two things to account for.

THE NITROGEN

Handle & excrete.

Amino groups
Liver: urea
Kidneys: urine

Nitrogen can be transferred between molecules and is ultimately disposed of largely through urea.

THE CARBON

Reuse or oxidize.

Carbon skeleton
Several metabolic entrances

Different amino acids enter as pyruvate, acetyl-CoA or cycle intermediates. Some are glucogenic, some ketogenic, and some both.

Simplified nitrogen and carbon routes. [22]

Protein can be both working material and fuel. Amino-acid oxidation does not wait until all carbohydrate and fat are gone. And a meal containing protein does not tell us how much becomes new muscle; synthesis, breakdown and the time scale all matter.

Galpin’s muscle series provides useful context. Watch his explanation of synthesis and breakdown .

08 / BUILD, STORE & RELEASE

Use some now.
Keep some moving.

The body is using, building and storing at the same time. What changes is the balance between those flows.

After a meal, insulin and nutrient availability generally favour glycogen and fat storage while restraining fat release. Between meals, stored fuel helps bridge the gap. A single meal has no fixed destination marked “visceral” or “subcutaneous”; tissue biology, hormones, blood flow and longer-term energy balance matter. [9][74]

THE SAME STORES · CHANGING FLOWS

A store is a two-way relationship.

Select a moment. See what changes without treating the fuels as a queue.

After a meal

Incoming nutrients support current work and replenish reserves. Insulin generally favours storage and restrains fat release.

Liver

Glucose → glycogen

Some arriving glucose can be stored; the liver also processes and shares nutrients.

Muscle

Glucose → local glycogen

Muscle can replenish its own working reserve while continuing to use ATP.

Adipose tissue

Fatty acids → triglyceride

Storage is favoured; fat release is generally reduced, rather than switched off everywhere.

A qualitative snapshot. Use and storage overlap; the balance varies with the meal, effort, hormones and the person. [9][39][74]

SUPPORT THE BLOOD SUPPLY

Liver glycogen

Glucose units stored in liver cells can be released as blood glucose between meals. The liver also makes glucose from precursors such as lactate and glycerol. [9][57]

LOCAL WORKING RESERVE

Muscle glycogen

Each muscle fiber uses its own glycogen. Muscle cannot directly export a useful supply of free glucose from that store; it can exchange other carbon-containing products, including lactate. [9][73]

A LARGER FUEL RESERVE

Adipose triglyceride

Fat cells store fatty acids joined to glycerol. Subcutaneous fat lies under the skin; visceral fat lies around abdominal organs. These are locations, not saturated or unsaturated fat types. [17][74]

MORE THAN FUEL

Working tissue & nutrient stores

Protein is active tissue, continually renewed. Vitamins and minerals have different stores: examples include B12 in the liver, calcium in bone and iron in ferritin. They are not all immediately discarded when unused. [22][30][75][77]

Stored fat can return to the blood.

Adipose triglyceride
Lipolysis
Fatty acids + albumin in blood
Uptake by tissues

Albumin is the blood protein carrying much of the released, non-esterified fatty-acid supply. It is not ALA: alpha-linolenic acid is one particular omega-3 fatty acid. Circulating fuel includes a mixture of fatty acids, not one universal “burning fat.” [16][73][79]

The liver also exports triglycerides in VLDL particles, and muscle has some local triglyceride stores. A released fatty acid may be oxidized or stored again. Working one muscle does not tell you which body-fat depot supplied it; regional contributions differ and can be studied with tracers. [42][73][80]

Trace a fuel through the original carbon explorer

A glucose route / 1 of 5

Single sugars

Digestive enzymes split starch and double sugars into absorbable single sugars. Glucose is the example followed here; fructose and galactose enter through extra processing.

Schematic routes; intermediates are omitted. Each path is one possible fate, not the fate of every molecule. [6][9][10][12][14][2]

Read all four pathways as text

Carbohydrate

Single sugars. Digestive enzymes split starch and double sugars into absorbable single sugars. Glucose is the example followed here; fructose and galactose enter through extra processing.
Pyruvate. In the cytosol, glycolysis changes one six-carbon glucose into two three-carbon pyruvates. It gives a small, rapid ATP return without directly using oxygen.
Acetyl-CoA. In mitochondria, pyruvate oxidation releases CO₂ and makes two-carbon acetyl groups. This is a shared junction with fat metabolism.
Krebs cycle. Continued cycling releases carbon as CO₂ and transfers electrons to carriers. The respiratory chain uses those electrons to support most ATP production.
CO₂ in the air. CO₂ travels from tissue through blood to the lungs. Its carbon came from fuel; it did not turn into ATP or heat.

Storage branch: glucose can also build glycogen. Liver glycogen supports blood glucose; muscle glycogen supplies its own muscle.

Fat

Fatty acids. Digestion makes dietary fat absorbable. For stored triglyceride, lipolysis releases fatty acids and glycerol. Mobilized fatty acids may be stored again.
Fatty acyl-CoA. Fatty acids are activated. Long-chain fatty acids use the carnitine shuttle to reach the mitochondrial matrix.
Acetyl-CoA. Beta-oxidation shortens the fatty-acid chain, usually two carbons at a time. It produces acetyl-CoA and electron carriers.
Krebs cycle. Acetyl-CoA enters the cycle. The cycle and respiratory chain connect fuel oxidation to ATP regeneration, CO₂ production and water formation.
CO₂ in the air. Much of the original fat mass ultimately leaves in exhaled CO₂. Some becomes water. Oxygen breathed in contributes to the product masses too.

Separate route: glycerol can enter carbohydrate metabolism. Common even-chain fatty acids do not yield a net gain of glucose in humans.

Protein

Amino acids. Digestion breaks proteins into amino acids and small peptides. Absorbed amino acids enter a pool used throughout the body.
Build or break down. Many amino acids become enzymes, transporters and other proteins. Those being broken down need separate handling for nitrogen and carbon.
Carbon skeletons. Nitrogen is handled largely through urea formation in the liver and excretion by the kidneys. Carbon skeletons enter at different metabolic junctions.
Several entrances. Depending on the amino acid, carbon may enter as pyruvate, acetyl-CoA or cycle intermediates. Some amino acids can support glucose production.
CO₂ + usable energy. When their carbon skeletons are oxidized, amino acids can contribute to ATP regeneration. Dietary protein does not automatically become muscle.

Building branch: amino acids are continually assembled into proteins and recycled. Muscle is working tissue, not an inert protein tank.

Fiber

Human digestion. Fiber resists digestion by human enzymes. It is a varied family, with different solubility, viscosity and fermentability.
The large intestine. Fiber that has escaped digestion reaches the colon. Not all of it is equally available to microbes.
Microbial work. Gut microbes ferment some fiber into short-chain fatty acids, including acetate, propionate and butyrate.
Short-chain fatty acids. These products can enter host metabolism. For example, butyrate is an important fuel for colon cells.
Fuel or stool. Fermented products can be used by the body. Other fiber contributes to stool. Fiber is neither all glucose nor all metabolically inert.

Different pathways: acetate, propionate and butyrate have different metabolic fates. This simplified view groups their contribution to the host.

09 / HOW CELLS MAKE ATP

Different fuels.
A shared chemistry.

Food breakdown can help regenerate ATP. Different fuels take different entrances into a connected network.

Some ATP is made outside mitochondria. In aerobic glucose oxidation, most comes from the machinery at the inner mitochondrial membrane. The Krebs cycle supplies that machinery with electron carriers.

A CELL’S ENERGY ROUTES · SIMPLIFIED

The cycle loads carriers.
The membrane makes most of the ATP.

01Cytosol · outside mitochondria

Split glucose

Glycolysis

This pathway can regenerate ATP without directly using oxygen. Pyruvate can enter mitochondria or become lactate. [10][60]

In: Glucose
Out: Pyruvate + a little ATP + NADH

Pyruvate enters mitochondria · fatty acids take their own entrance

INSIDE A MITOCHONDRION

02Mitochondrial matrix

Prepare the entrance

Pyruvate oxidation / beta-oxidation

Pyruvate processing releases CO2. Fatty-acid breakdown supplies acetyl-CoA by a different route. Amino-acid carbon can enter at several points. [12][18][50]

In: Pyruvate or fatty-acid breakdown products
Out: Acetyl-CoA + electron carriers

Acetyl-CoA

03Mitochondrial matrix

Turn the cycle

Krebs / citric acid / TCA cycle

A sequence of reactions returns to its starting acceptor. Much of the captured energy leaves on electron carriers. The cycle also supplies ingredients for synthesis. [12][13]

In: Acetyl-CoA joins a regenerated acceptor
Out: CO2 + NADH / FADH2 + a little ATP equivalent

Electron carriers connect these reactions to the membrane

04Inner mitochondrial membrane

Use the gradient

Electron transport + ATP synthase

Electron transport pumps protons across the membrane. Their return through ATP synthase helps make ATP from ADP and phosphate. Oxygen accepts electrons at the chain’s end and is reduced to water. [14]

In: Electrons from carriers + oxygen
Out: Most ATP in aerobic glucose oxidation + water

NADH and FADH2 are electron carriers. Acetyl-CoA carries an acetyl group into metabolism. The compartments are schematic; the diagram does not track individual atoms or show every reaction.

Why a cycle?

The starting acceptor, oxaloacetate, is regenerated. New acetyl groups can enter on later turns. Carbon also moves into and out of the cycle for other jobs. [12][13]

Where does oxygen go?

Oxygen is reduced to water at the respiratory chain. CO2 is released in carbon-removing reactions, including pyruvate oxidation and the cycle. It is not a final pile of leftover carbon simply mixing with oxygen. [12][14]

Where do micros fit?

B2 contributes to FAD; B3 to NAD; B5 to coenzyme A. Nutrients help the machinery work without themselves supplying calories. Extra vitamins are not an automatic accelerator. [24][25][26]

A CONNECTED BRANCH

Lactate keeps carbon moving.

Converting pyruvate to lactate regenerates NAD+ so glycolysis can continue. This happens even when oxygen is available. Lactate can travel to another tissue, become pyruvate again and be oxidized, or supply carbon for liver glucose production. [60][61]

Pyruvate ↔ lactate
Blood → other tissues
Oxidation / new glucose

The liver route is part of the Cori cycle and costs energy. Lactate formation itself does not add extra ATP beyond glycolysis.

THREE CONNECTED EXPERIMENTS

Follow carbon. Turn the cycle. Reuse lactate.

Krebs cycle, citric acid cycle and TCA cycle name the same pathway. An acetyl group joins a four-carbon acceptor; the cycle releases CO₂, loads carriers and restores that acceptor.

Lactate is a separate, connected branch: its formation regenerates NAD⁺ for glycolysis, and its carbon can be reused. [12][13][60][61]

ATP synthase illustrated as a membrane-spanning molecular machine, with a shaft connecting its membrane portion to a large catalytic head. — ATP synthase illustration by David S. Goodsell / RCSB PDB, CC BY 4.0. Resized and converted to WebP; no other changes.

Go deeper: reaction stages and the complete cycle diagram

The Krebs cycle is the meeting point in the story. It is not the whole story.

To see the connections, follow one free glucose molecule through glycolysis, pyruvate oxidation, the cycle, and oxidative phosphorylation. Notice where carbon leaves and where electrons move. Those are related but different events.

Cytosol

Six carbons. Two threes.

Glucose (6C) → 2 pyruvate (3C each)

A short investment pays a small return: two ATP are spent and four are made. Two NADH carry electrons onward. No CO₂ leaves in this step.

DIRECT YIELD: 2 net ATP
ELECTRON CARRIERS: 2 NADH
CARBON EXIT: 0 CO₂
OXYGEN: No direct O₂ use

Pyruvate can also become lactate, regenerating NAD⁺. That reaction can happen with oxygen present. Lactate can be transported and used as fuel; it is not simply a waste product. [10][60][62]

Read all four stages as text

Glycolysis · Cytosol

A short investment pays a small return: two ATP are spent and four are made. Two NADH carry electrons onward. No CO₂ leaves in this step. Pyruvate can also become lactate, regenerating NAD⁺. That reaction can happen with oxygen present. Lactate can be transported and used as fuel; it is not simply a waste product.

2 net ATP; 2 NADH; 0 CO₂; No direct O₂ use. [10][60][62]

Pyruvate oxidation · Mitochondrial matrix

Each pyruvate loses one carbon as CO₂. Its remaining two-carbon acetyl group joins coenzyme A. NADH also forms at this gateway. This gateway is not glycolysis, and it is not yet the Krebs cycle. Keeping it separate makes the carbon accounting much easier to follow.

0 direct ATP; 2 NADH; 2 CO₂; O₂ used downstream. [12]

Krebs cycle · Mitochondrial matrix*

An acetyl group joins oxaloacetate to form citrate. Reactions regenerate oxaloacetate, release CO₂ and load electron carriers. Two turns account for one glucose. The two carbons entering a turn are not generally the same two leaving in that first turn. These are net counts across continued cycling. *Succinate dehydrogenase is embedded in the inner membrane.

2 GTP / ATP equivalents; 6 NADH · 2 FADH₂ equivalents; 4 CO₂; O₂ used downstream. [12][13]

Oxidative phosphorylation · Inner mitochondrial membrane

Electrons pass through the respiratory chain. Energy from that transfer moves protons across the membrane. Their return through ATP synthase drives ATP production. Oxygen accepts electrons and becomes water. The whole glucose journey gives roughly 30–32 ATP in this teaching model. Shuttle pathways, proton leak and transport costs change the yield. It is an estimate, not a fixed biological invoice.

Most of the ATP; NAD⁺ & oxidized carriers recycled; No CO₂ at ATP synthase; O₂ → water. [14][50]

Inspect the full cycle diagram

Eight intermediates, one returning route.

Citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate.

Per acetyl-CoA: 2 CO₂, 3 NADH, 1 FADH₂ equivalent and 1 GTP/ATP equivalent. [12][13]

ATP is continually rebuilt.

ATP hydrolysis can be coupled to cellular work. The overall reaction releases free energy; breaking a chemical bond by itself costs energy. Food’s “4, 4, 9” values—roughly 4 kcal/g of digestible carbohydrate or protein and 9 kcal/g of fat—are average food-energy factors, not ATP counts. [3][4]

Lactate belongs on the map.

Converting pyruvate to lactate regenerates NAD⁺ so glycolysis can continue. [60] Lactate is exchanged among tissues, can be oxidized, and can contribute carbon to new glucose. [57][61] It is useful fuel, including for the human brain under studied conditions. [11]

10 / TOMORROW'S ENERGY

Tomorrow, climb a hill.
What powers the work?

Tomorrow you climb a hill. Your muscles need ATP immediately—and keep needing it for every step.

Stored ATP is a small working supply. Phosphocreatine rapidly transfers a phosphate to ADP to rebuild ATP. Glycolysis and oxidative metabolism also contribute from the start; their rates change with intensity, duration, training and available fuel. These are overlapping systems, not four tanks emptied in order. [72][73]

How the emphasis changes · qualitative, not measured fuel percentages
Situation	Phosphocreatine	Glycolysis	Oxidative metabolism
A sudden hard effortLift, jump, accelerate	Rapid ATP buffering	Rises quickly	Already contributing; ramps up
Sustained hard workA hard climb or interval	Small reserve; must be rebuilt	Large contribution from carbohydrate	Substantial contribution alongside glycolysis
Steady easier movementWalk or comfortable ride	Buffers changes in demand	Continues supplying pyruvate	Often supplies most ATP; mixes carbohydrate and fat
Recovery & the next mealRest, eat, replenish	Replenished using ATP	Continues at a lower demand	Supports recovery; fuel mixture keeps changing

Fat oxidation contributes during easier movement before glycogen runs out. Harder work generally relies more on carbohydrate; there is no universal minute when “fat burning starts.” [72][73]

Two carbohydrate supplies

Blood glucose enters muscle from the circulation. Local muscle glycogen is already inside muscle cells. Both can feed glycolysis; they are different sources feeding a shared pathway. [9][73]

Several fat supplies

Muscle can use blood-borne fatty acids and its own lipid stores. Fatty-acid oxidation feeds mitochondrial metabolism; it cannot match every sudden rise in ATP demand on its own. [73]

What is the “burn”?

The sensation during hard effort is not a gauge of local fat loss or a direct measure of glycogen use. Sensory nerves respond to a combination of chemical changes. Lactate alone does not explain it. [81]

Activity ends; metabolism continues. ATP helps restore phosphocreatine, meals help replenish glycogen, and tissue renewal continues. [72][73]

Compare organ responses after a meal, between meals and during movement

Metabolism is coordinated across organs. The same molecule can have different destinations in the liver, muscle, brain or fat tissue.

Hormones and cellular demand influence the rates of pathways. A meal, the time between meals, and movement change which routes become more prominent. They do not create three mutually exclusive metabolisms.

Incoming nutrients, many destinations.

Absorption rises, insulin often rises, and building and storage become more prominent. Cells keep using fuel throughout.

Liver

Processes incoming sugars; glycogen synthesis often rises.

Muscle

Uses ATP and can replenish glycogen or build protein.

Adipose tissue

Stores triglyceride; insulin suppresses fat mobilization.

Insulin supports GLUT4-mediated glucose uptake in muscle and adipose tissue. It is not the entry key used by every cell.

Qualitative teaching model. No universal fuel percentages or transition times are implied. [39][43]

Some cells have different options.

Red blood cells have no mitochondria, so they rely on glycolysis for ATP. The brain usually relies heavily on glucose, but can also use lactate and, when available in sufficient amounts, ketone bodies. [10][11][40]

Heat belongs in the accounting.

Brown and beige fat can use UCP1 to let the mitochondrial proton gradient dissipate as heat. This helps explain thermogenesis; it does not by itself establish a fat-loss result from a cold-exposure routine. [46][47]

Does metabolism just slow down every year?

Energy use includes resting expenditure, movement and food processing. Absolute needs change with body size, composition and behavior. A large 2021 study found broadly stable expenditure adjusted for body composition from about age 20 to 60, followed by a later decline. That is not a promise that every person’s unadjusted calorie needs stay fixed for four decades. [48][49]

11 / BREATH & METABOLISM

The surprising exit
is an ordinary breath.

Stored fat contains carbon, hydrogen and oxygen. Oxidizing it produces carbon dioxide and water.

CO₂ made in your tissues travels through blood, much of it temporarily carried as bicarbonate. At the lungs, it becomes CO₂ gas again and moves into the air sacs to be exhaled. Metabolic water joins the water already in the body. [1][2]

That is the meaning of “breathe out fat.” It describes a material exit after cells process fuel. Ventilation removes CO₂; it does not command fat cells to empty.

FOLLOW THE CARBON EXIT

From a working cell to the air.

CO2 made in tissues
Blood · mostly bicarbonate
Lung air sacs
Exhaled CO2

Oxygen travels inward. Lungs → blood → tissues → the respiratory chain.

Carbon dioxide travels outward. Carbon-removing reactions → blood → lungs.

Energy changes form. Fuel oxidation supports ATP regeneration, cellular work and heat release.

These are linked routes, not an instruction to breathe faster to lose fat. [1][2][14]

An abstract amber and sage ribbon opening into an airy plume; a visual motif for outward flow, not lung anatomy. — **The route from tissue to air.**Conceptual flow illustration; the ribbon is a visual metaphor, not visible CO₂, a blood vessel or lung anatomy.

FOLLOW THE EXIT

Carbon leaves a reaction.

Oxidative metabolism produces CO₂ in tissues. This is one fate of fuel carbon; other carbon can remain in body structures or stores.

Steps are simplified. The animation shows no physiological rate, quantity or breathing instruction. [2][12]

Follow the mass: fat, oxygen, carbon dioxide and water

Nothing disappears.

A simplified triglyceride oxidation model.

Include inhaled oxygen

Triglyceride oxidized 10 kg

1 kg10 kg

8.4 kg

Original fat mass in CO₂

Leaves through the lungs in your breath.

1.6 kg

Original fat mass in water

Joins the body’s water pool.

84% → CO₂

16% → H₂O

The bar tracks original fat mass only. Rounded accounting: 10 kg fat + 29 kg O₂ ≈ 28 kg CO₂ + 11 kg water. Product mass includes inhaled oxygen. Educational model, not a body-weight prediction. [1] Figure correction.

BACK TO THE WHOLE BODY

This whole exchange is metabolism.

A meal supplies molecules. Digestion and absorption make them available; organs distribute and transform them. Cells build, store, release and oxidize material while continually regenerating ATP. Tomorrow’s movement draws on that changing supply—and some of the carbon eventually leaves in your breath.

Metabolism includes all of this: the building as well as the breakdown, the work as well as the heat, the stored material as well as the material leaving.

Return to the ingredients

12 / CHECK YOUR UNDERSTANDING

Can you connect
the dots?

100 questions across 10 levels. Take ten at a time, connect the ideas, and keep a notebook of what clicked and what needs another look.

Connect the whole journey.

Practice with ten questions at a time, or choose a deeper challenge from the 100-question bank. Each answer explains the connection.

The complete reader also includes every question and answer.

BACK TO THE WHOLE STORY

The carbon in a meal might become part of you.
It might also leave in your next chapter of breaths.
Now you can follow the journey.

13 / SOURCES & FIELD NOTES

Good questions
have a backstory.

This primer grew from Shreyam Adhikari’s two-year-old notes and Dr. Andy Galpin’s physiology lessons.

The lessons below supplied the original learning framework. The refined explanations were checked against physiology texts, official nutrient references and original studies. This is an independent article; Galpin has not reviewed or endorsed it.

THE ORIGINAL GOOGLE DOCRead the metabolism field notesThe source that started this article.

The Andy Galpin reading room.

Original lessons

Scientific references · 83 entries

Nutrient references

About this edition, illustration & design

Made from two-year-old notes by Shreyam Adhikari, originally written around 2024 and kept in a personal Google Docs archive. Recovered on 9 September 2026; last refined on 11 September 2026. The original notes remain linked above. First published in Shreyam’s log on 9 September 2026.

The amber artwork and mixed-meal illustration were generated for this article. The amber artwork is a conceptual illustration, not a molecular model. Foods contain mixtures of nutrients; the meal illustration is not portion guidance. Interactive pathways are simplified teaching diagrams; their qualifications appear alongside them.

Design references include the stepwise explorations of Bartosz Ciechanowski and the learning-through-interaction approach collected at Explorable Explanations. The page layout and diagrams were created for Carbon Atlas. The dimensional scenes also take inspiration from Kage; its artwork and code are not reused. The molecular viewers use 3Dmol.js (BSD-3-Clause) and CC0 coordinates from RCSB PDB, credited beneath each model. The protein uses the complete deposited atoms of one NMR model; its ribbon is a structural guide. Still images are rendered from the same data. The conceptual hero uses Three.js (MIT). The fiber and breath artwork was generated for this article with OpenAI ImageGen, with scientific labels and qualifications written separately. Gentle image movement is illustrative, not a simulation. Artwork prompts, sources and scope. Fonts: Newsreader and DM Sans (SIL Open Font License). Icons: Phosphor (MIT).

To retain the structure behind the page, download the refined article or download the nutrient atlas.

Keep a copy of the article Back to the beginning

First, meetthe ingredients.

Macronutrients

Micronutrients

One plate.Many kinds of molecule.

The machinery needsmore than fuel.

The nutrient atlas.

01Water & choline

Water + choline

02Carbohydrates

One sugar unit

Two sugar units

Broader families

Body store

03Fats & lipids

Molecular structure

Double-bond count

Bond geometry

Essential fatty acids

04Proteins & amino acids

The family

Nine indispensable amino acids

Eleven the body can usually make

The qualification

05Vitamins

Fat-soluble · A, D, E, K

Water-soluble · eight B vitamins + C

06Minerals & electrolytes

Major dietary minerals

Established trace minerals

07Plant compounds

Phytochemical families

08Special cases & body-made compounds

Elemental context

Made in the body

Length is one question.

Labels can overlap.

9 + 11 = 20.

Where do salts and electrolytes fit?

A family can cross the map.

The label matters.

Water & choline

Fat-soluble vitamins

Water-soluble vitamins

Major dietary minerals

Established trace minerals

Indispensable amino acids

Essential fatty acids

Carbohydrate families

Protein family

Lipid families & structures

Selected plant-compound families

Mineral classification details

Synthesis & conditional needs

Monosaccharides · single sugars

Disaccharides · paired sugars

Other standard amino acids

Energy changes form.Matter goes somewhere.

What changes along the way?

Follow the starch in a meal.

Joined glucose units

Chains become sugars

Across the intestinal lining

Two ways into circulation.

Several possible destinations.

Use and build

Liver glycogen

Muscle glycogen

A meal enters.Different routes open.

Break down. Then cross the wall.

Mouth

Stomach

Small intestine

Colon

Does everything go to the liver first?

Sugar, starch, fiber.One family. Different paths.

One sugar. A pair. A chain.

Six carbons. A shape in space.

α-D-glucopyranose

Spend it now.

Keep it as glycogen.

First, meet
the ingredients.

One plate.
Many kinds of molecule.

The machinery needs
more than fuel.

Energy changes form.
Matter goes somewhere.

A meal enters.
Different routes open.

Sugar, starch, fiber.
One family. Different paths.

A compact store.
A different entrance.

From amino acids
to working proteins.

Use some now.
Keep some moving.

Different fuels.
A shared chemistry.